Are waking fantasies that include marital interactions with a fiancé (whom one has not yet entered into marriage with) considered among "the transgressors" mentioned in His — Most High — saying: "But whoever seeks beyond that, then those are the transgressors"? And what are the implications of that?

If a man contracts a valid marriage with a woman, she becomes his wife, and it becomes permissible for them to enjoy each other. There is no harm in imagining the husband engaging in intercourse with his wife as long as the contract is valid. This does not fall under the meaning of Allah's saying: "But whoever seeks beyond that, then those are the transgressors" (Qur'an 23:7). However, one should be cautious that this does not lead to masturbation. It is advised to seek Allah's help and to occupy one's time with what benefits them in their religious and worldly affairs, alongside frequent remembrance of Allah (dhikr) and supplication (du'a).